The present invention relates to a pervaporation process and to membranes useful in such a process.
Pervaporation is a separation process in which a liquid feed mixture to be separated into components is brought into contact with a first ("feed") side of a suitable membrane. If, at the second ("permeate") side of the membrane, the partial vapor pressures of the components of the liquid feed mixture are kept below the partial vapor pressures of the components at the feed side, a driving force is established for the migration of those components through the membrane. In this regard, the permeation rate ("permeability") of each component is a complex function of a number of different parameters, such as the nature and concentration of the permeating species, the nature and structure of the membrane, the composition of the mixture, temperature, and others.
Based on their respective permeabilities, the components will pass through the membranes at different rates. Since the partial vapor pressures are lower at the permeate side than at the feed side of the membrane, the components will evaporate after passing through the membrane and form a vaporous permeate, which is why the process is called "pervaporation." Due to the different permeabilities of the components, the composition of the permeate will differ from the composition of the feed mixture, and a separation of the feed mixture is observed.
So long as a sufficiently large difference in partial vapor pressures is maintained between the feed and permeate side, the separation capacity of a given membrane toward a given feed mixture is determined solely by the permeabilities of the respective components. Only nonporous membranes can be used for pervaporation, and it is believed that solubility of a component in the membrane material, together with diffusivities, govern mass transport across the membrane. Pervaporation is thus a nonequilibrium dynamic process, where transport phenomena determine separation efficiency.
The membrane-based process of pervaporation is generally known, and has been described in the literature, for instance, by M. H. V. Mulder et al., J. Membr. Sci. 16: 269 (1983), and by G. F. Tusel et al., Desalination 53: 327 (1985). Other pervaporation membranes are described by J. Neel et al., Desalination 53: 297 (1985), and by J.-P. Brun et al., J. Membr. Sci. 25: 55 (1985).